1. Field of the Invention
The present invention relates to an ESD protection circuit and particularly to an early triggered MOSFET ESD protection circuit.
2. Description of the Prior Art
An NMOS transistor can be used as an ESD protection device. In one application, with the gate connected to the gate-driving signal, the NMOSFET is used as the pull-down device of a CMOS buffer to drive the output voltage. In another application, with the gate electrically coupled to the ground, the NMOSFET is used to protect an input pin or power bus during an ESD event.
The ESD protection of an NMOSFET heavily depends on the snapback mechanism for conducting large ESD currents between the drain and source. At the beginning, the high electric field at the drain junction causes impact ionization with generation of both minority and majority carriers. The minority carriers are collected at the drain (anode), while the majority carriers flow toward the substrate or p-well contact (cathode) causing a local potential build up in the p-well. When the local substrate potential is 0.8V higher than the adjacent n+ source potential, the source junction becomes forward biased. The forward biased source junction injects minority carriers into the p-well. Some of those injected minority carriers are recombined in the substrate while the rest reach the drain junction to further enhance the impact ionization. As a continuous loop, the MOSFET enters a low impedance (snapback) state to conduct large ESD currents.
FIG. 1 is a diagram showing a conventional gate coupled MOSFET ESD protection circuit. The gate potential of the shunt transistor N1 is raised to approximately 1 to 2 volts during a positive-voltage ESD event, to reduce the ESD trigger voltage and enhance the uniform turn on of a multiple-gate-finger NMOSFET.
U.S. Pat. No. 6,304,127, 6,091,593 and 5,870,268 described methods of generating a negative transient voltage during a positive-voltage ESD event. The negative transient voltages were applied to a diffusion or well region to trigger an ESD protection device early, by either increasing the terminal voltage difference, or causing a forward-biased junction for carrier injection.